docs: def background_at_centroid(self):
          """
          The value of the ``background`` at the position of the source
          centroid.
  
          The background value at fractional position values are
          determined using bilinear interpolation.
          """
  
          from scipy.ndimage import map_coordinates
  
          if self._background is not None:
              # centroid can still be NaN if all data values are <= 0
              if (self._is_completely_masked or
                      np.any(~np.isfinite(self.centroid))):
                  return np.nan * self._background_unit  # unit for table
              else:
                  value = map_coordinates(self._background,
                                          [[self.ycentroid.value],
                                           [self.xcentroid.value]], order=1,
                                          mode='nearest')[0]
  
                  return value * self._background_unit
          else:
              return None